Method for Removing/Concentrating Metal-Fog-Forming Metal Present in Molten Salt, Apparatus Thereof, and Process and Apparatus for Producing Ti or Ti Alloy by use of them

ABSTRACT

The present invention provides a method by which a metal-fog-forming metal dissolved in one portion of “a molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts” (generally, a molten salt) can be removed and transferred to another portion of the molten salt to increase the concentration thereof. The method can hence be utilized as one of means for treating molten salts in various industrial fields in which metal-fog-forming metal-containing molten salts such as Ca or Na are handled. In particular, when the method is utilized in producing Ti by Ca reduction, the Ca dissolved in the molten salt to be fed to an electrolytic cell can be rapidly removed (recovered) and the Ca formation efficiency during the electrolysis of the molten salt can be enhanced. Consequently, Ca formation and TiCl 4  reduction in the electrolysis of the molten salt can be efficiently carried out and a stable operation on a commercial scale is possible. Thus, the method can be efficiently utilized in producing Ti or a Ti alloy by Ca reduction.

TECHNICAL FIELD

The present invention relates to a method of removing/concentrating a metal-fog-forming metal, for example Ca or Na, dissolved in a molten salt containing such a metal-fog-forming metal as a constituent thereof, the method comprising removing the metal-fog-forming metal from the molten salt and transferring the same to other metal-fog-forming metal containing molten salt to increase the concentration thereof, and to an apparatus therefor, as well as to a process for producing Ti or a Ti alloy by use of the above method in carrying out the reduction treatment of a TiCl₄-based metal chloride mixture with Ca to produce Ti or a Ti alloy, and to an apparatus therefor.

BACKGROUND ART

Such metals as Ti, Zr, Ta, Hf and V are useful metals respectively having desirable characteristics but can be hardly refined using such a conventional reducing agent as C or Al. It is necessary to separate those metals from co-existent congeners and from impurities and, therefore, they are generally produced by refining through a number of steps such as solvent extraction, roasting and chlorination, followed by conversion to oxides or chlorides, further followed by reduction thereof with a strong reducing agent such as Mg, Al, Na or Ca.

Reducing agents such as Ca, Na and Al and, further, Li, by nature, are themselves soluble in metal chlorides (e.g. Ca is soluble in CaCl₂) and, on the occasion of dissolution, they produce a foggy matter called “metal fog”. Such metals are referred to herein as “metal-fog-forming metals”.

These metal-fog-forming metals are refined from raw material ores through various refining treatments to give their respective pure forms for use in various fields of application, including the use of such reducing agents as mentioned above. On the other hand, the chlorides and fluorides, among others, of these metals are often used also in the form of molten salts either alone or in multi-component systems containing another salt or other salts and, in particular, they are widely used as industrial electrolytic baths in molten salt electrolysis.

Further, concerning the utilization of these metal-fog-forming metals, the technology which uses Ca as a reducing agent in producing Ti from TiCl₄ has been proposed. A conventional commercial process of producing Ti which can utilize such a metal-fog-forming metal is the Kroll process according to which TiCl₄ is reduced with Mg.

In the Kroll process for commercial production of Ti, metallic titanium is produced via a reduction step and a vacuum separation step. In the reduction step, TiCl₄ in liquid form, fed to a reaction vessel from above, is reduced by molten Mg to form particles of metallic Ti, which successively move downward to give spongy metallic Ti. In the vacuum separation step, the unreacted portion of Mg and the by-product MgCl₂ are removed from the spongy metallic Ti occurring in the reaction vessel.

In the production of metallic Ti by the Kroll process, it is possible to produce high-purity products. Since, however, it is a batch-wise process, the production costs add up and product prices become very high. One of the causes of increased production costs is a difficulty in increasing the feeding rate of TiCl₄.

Several possible reasons therefor are conceivable. One is that, at a high TiCl₄ feeding rate exceeding a certain limit, the TiCl₄ fed from above onto that portion of MgCl₂ which has not yet moved downward but remains on the liquid surface is partly discharged out of the reaction vessel in the form of unreacted TiCl₄ gas and insufficiently reduced TiCl₃ gas, among others, and the TiCl₄ utilization efficiency decreases accordingly.

Further, in the Kroll process, the reaction occurs only in the vicinity of the molten Mg liquid surface in the reaction vessel, hence the heat release area is narrow. Therefore, cooling cannot keep up with such high-rate feeding of TiCl₄, which is another major reason for the TiCl₄ feeding rate being restricted.

Furthermore, the Ti powder formed moves downward in an aggregated state due to wettability (viscosity) of the molten Mg and, during moving downward, grains thereof are sintered and grown due to the heat which the high-temperature melt harbors, making it difficult to recover them out of the reaction vessel. Therefore, the metallic Ti production cannot be performed in a continuous manner; hence productivity is impaired.

As for Ti production processes other than the Kroll process, U.S. Pat. No. 2,205,854 describes that Ca, for example, can be used instead of Mg as an agent for reducing TiCl₄. And, U.S. Pat. No. 4,820,339 describes a process for producing Ti by utilizing reduction reaction with Ca which comprises maintaining a CaCl₂-based molten salt in a reaction vessel, feeding a metallic Ca powder to the molten salt from above to allow the Ca to be dissolved in the molten salt while feeding TiCl₄ gas from below to thereby cause the molten Ca to react with the TiCl₄ in the CaCl₂-based molten salt.

However, the process described in the above-cited U.S. Pat. No. 4,820,339 cannot serve as a commercial process of producing Ti since the metallic Ca powder to be used as the reducing agent is very expensive and, when this is purchased, the production cost will become higher as compared with the Kroll process. In addition, Ca, which is strongly reactive, is very difficult to handle; this fact is also major factor inhibiting the commercial Ti production by reduction with Ca.

As a further process of producing Ti, U.S. Pat. No. 2,845,386 describes the so-called Olson process in which TiO₂ is directly reduced with Ca, not via TiCl₄. This process is a kind of direct oxide reduction process. However, it is necessary to use expensive high-purity TiO₂ in this process.

On the other hand, the present inventors, who considered it necessary, for establishing a commercial process for producing Ti by reduction with Ca, to supplement Ca consumed in the reduction reaction to the molten salt in an economical manner, proposed, in Japanese Patent Application Publication Nos. 2005-133195 and 2005-133196, a process, namely the so-called “OYIK process”, according to which Ca formed upon electrolysis of molten CaCl₂ is utilized and this Ca is recycled.

The process described in the above-cited Japanese Patent Application Publication No. 2005-133195 comprises introducing a Ca-rich molten CaCl₂ supplemented with Ca formed by electrolysis into a reaction vessel for use in formation of Ti particles by reduction with Ca, and the process described in Japanese Patent Application Publication No. 2005-133196 further comprises using an alloy electrode (e.g. Mg—Ca alloy electrode) as the cathode for effectively suppressing the back reaction in association with the electrolysis.

DISCLOSURE OF THE INVENTION

As mentioned above, a number of research and development have so far been made in search of Ti production processes other than the Kroll process. In the OYIK process proposed by the present inventors, in particular, Ca in the molten salt is consumed as the reduction reaction of TiCl₄ proceeds and, when the resulting molten salt is electrolyzed, Ca is formed in the molten salt, and the reuse of the thus-obtained Ca for the reduction reaction makes it unnecessary to supplement Ca from the outside and further makes it unnecessary to take out Ca alone, so that the economical efficiency is improved.

Therefore, the present inventors attempted to develop a metallic Ti or Ti alloy production process whose basic constitution is based on the OYIK process and which can be carried out efficiently and stably on a commercial scale and made investigations on all the production steps in that process.

It is an object of the present invention, which has been made in view of the fact that such metal-fog-forming metals as Ca and Na are widely used as constituents in various molten salts, typically industrial electrolytic baths, to provide a method of removing/concentrating a metal-fog-forming metal in a molten salt according to which a metal-fog-forming metal (Ca) can be removed from a molten salt composition comprising a molten salt (e.g. CaCl₂) containing a metal-fog-forming metal such as Ca or Na as a constituent thereof, with the metal-fog-forming metal (Ca) dissolved therein, and, at the same time, the thus-removed metal-fog-forming metal can be transferred to another molten salt composition (molten salt composition containing the metal-fog-forming metal as a constituent thereof), as well as an apparatus for use in carrying out such method.

It is a further object of the present invention to provide a Ti or Ti alloy production process according to which the reduction of TiCl₄ or another metal chloride and, further, the formation of Ca by molten salt electrolysis are effected efficiently in the production of metallic Ti or a Ti alloy by reduction of TiCl₄ or the other metal chloride with Ca formed by electrolysis of molten CaCl₂ and which can be performed stably on a commercial scale, as well as a production apparatus suited for that process.

The present invention has been made to accomplish the objects mentioned above and is now described in its three aspects, namely “1. method of removing/concentrating a metal-fog-forming metal in a molten salt, and apparatus therefor”, “2. process for producing Ti or a Ti alloy which includes a Ca recovery step, and apparatus therefor” and “3. process for producing Ti or a Ti alloy which includes a Ca removing/concentrating step, and apparatus therefor”.

1. Method of Removing/Concentrating a Metal-Fog-Forming Metal in a Molten Salt, and Apparatus Therefor

The present inventors made investigations concerning the case that the metal-fog-forming metal is Ca. As a result, it was found that when the molten salt (CaCl₂) is held in a treatment vessel (hereinafter, referred to as “vessel A”) and a Ca-containing molten alloy (a molten Mg—Ca alloy) is held thereon in contact with, the molten salt and a voltage below the decomposition voltage for CaCl₂ is applied so that the electrode plate on the molten alloy side may serve as a negative (−) electrode and the electrode plate on the molten salt side as a positive (+) electrode, the Ca dissolved in the molten salt can be rapidly absorbed into the molten alloy, thus being removed.

When, on the contrary, a voltage below the decomposition voltage for CaCl₂ is applied so that the electrode plate on the molten alloy side may serve as a positive (+) electrode and the electrode plate on the molten salt side as a negative (−) electrode (hereinafter such vessel is referred to as “vessel B”), the Ca is transferred from the molten alloy side to the molten salt side.

It was further found that when the molten alloy as a common constituent for the above vessel A and vessel B is connected to be integrated and a voltage (voltage below the decomposition voltage for CaCl₂) is applied so that the electrode plate on the molten salt side in vessel A may serve as a positive (+) electrode and the electrode plate on the molten salt side in vessel B as a negative (−) electrode, the removal (absorption into the molten alloy) of the Ca dissolved in the molten salt in vessel A and the transfer of the Ca from the molten alloy side to the molten salt side in vessel B can be effected simultaneously. Namely, it was revealed that the removal of the Ca dissolved in the molten salt on one side and the enrichment of the Ca dissolved in the molten salt on the other side can be caused to proceed simultaneously and rapidly by appropriate voltage application.

And, it has been confirmed that when such a method as mentioned above is applied to the above-mentioned OYIK process proposed by the present inventors, it is possible to allow the formation of Ca by electrolysis to proceed efficiently while suppressing a so-called back reaction, which otherwise readily occurs in the step of electrolysis of a molten CaCl₂, and, further, to effectively enhance the efficiency of the reduction reaction of TiCl₄ (formation of Ti particles by reduction of TiCl₄ with Ca) as well.

In applying this method to the OYIK process, all the production steps of the process have been reexamined so as to carry out the operations efficiently and stable on a commercial scale and, as a result, a markedly improved and advanced modification of the OYIK process has been newly developed; the modification can be regarded as a new development of the OYIK process and may be referred to, for example, as “OYIK-II process”.

The method of removing/concentration a metal-fog-forming metal in molten salt and the apparatus therefor according to the present invention have been developed based on such findings and re-examination results as mentioned above and respectively have constitutional features (1) and (2) described below.

(1) A method of removing/concentrating a metal-fog-forming metal in molten salt, the method comprising: holding a molten salt in a metal-fog-forming metal concentrating and removing regions, being apart from each other, inside a metal-fog-forming metal removing/concentrating vessel, the molten salt being a molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts, the metal-fog-forming metal being dissolved therein; holding a metal-fog-forming metal containing molten alloy in contact with the molten salt held in each of the metal-fog-forming metal concentrating and removing regions; and applying a voltage below a decomposition voltage for the metal-fog-forming-metal containing molten salt so that an electrode plate on the molten salt side within the metal-fog-forming metal removing region may serve as a positive (+) electrode against the molten salt side within the metal-fog-forming metal concentrating region, thereby causing the metal-fog-forming metal dissolved in the molten salt within the metal-fog-forming metal removing region to be absorbed into the molten alloy, resulting in the decrease in concentration thereof and, at the same time, causing the metal-fog-forming metal dissolved in the molten salt within the metal-fog-forming metal concentrating region to be highly concentrated, resulting in the increase in concentration thereof.

The “metal-fog-forming metal” so referred to herein is a metal strong in reducing power, soluble itself in corresponding metal chloride (e.g. Ca being soluble in CaCl₂) and capable of forming a fog called a metal fog on the occasion of dissolution, such as Ca, Li, Na or Al.

The term “a metal-fog-forming-metal containing molten salt” as used herein refers to the molten salt in which a metal-fog-forming metal is contained as a constituent thereof, for example a molten CaCl₂ or a molten NaCl. “A molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts”, should mean, in the case of the metal-fog-forming metal being Ca, for instance, either a molten CaCl₂ alone or a molten salt mixture comprising a molten CaCl₂ and CaF₂ or the like that is added for lowering the melting point and adjusting the viscosity, for instance.

The “a metal-fog-forming metal containing molten alloy” is an alloy in a molten state with the metal-fog-forming metal being contained therein as a constituent thereof and, in the case of the metal-fog-forming metal being Ca, for instance, it indicates a molten Mg—Ca alloy, a molten Pb—Ca alloy or the like.

The electrodes between which a voltage is to be applied are designated as “negative (−) electrode” and “positive (+) electrode”, as so referred to hereinabove, for avoiding the confusion thereof with the terms “anode” and “cathode”, which are used on the premise that a salt bath (herein, molten salt) is electrolyzed.

The method of removing/concentrating a metal-fog-forming metal according to the present invention as described above under (1) can be carried out in a mode of embodiment in which the metal-fog-forming metal is Ca and the metal-fog-forming metal containing molten salt is a Ca-containing molten salt and, further, in a desirable mode of embodiment in which the metal-fog-forming metal is Ca and the metal-fog-forming metal containing molten salt is CaCl₂. The “Ca-containing molten salt” just mentioned above refers to CaCl₂ or CaF₂, for instance.

When, in the above mode of embodiment, the applied voltage is lower than 3.2 V, it is possible to cause Ca to be dissolved in molten alloy without causing decomposition of CaCl₂ while controlling the voltage to be applied at specific numerical value levels.

(2) An apparatus for removing/concentrating a metal-fog-forming metal in molten salt, comprising a metal-fog-forming metal removing/concentrating vessel having a for metal-fog-forming metal concentrating and removing regions and a holding region for molten alloy, wherein the concentrating region is for holding a molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts, the molten salt mixture having an enriched metal-fog-forming metal that is dissolved therein, and wherein the removing region is for holding a metal-fog-forming metal containing molten salt that has a rarified metal-fog-forming metal as being dissolved therein as a result of application of a voltage below the decomposition voltage for the metal-fog-forming metal containing molten saltvia an electrode plate so that it may serve as a positive (+) electrode against the molten salt side within the metal-fog-forming metal concentrating region, and wherein the holding region is for holding a metal-fog-forming metal containing molten alloy in contact with the molten salt held in the metal-fog-forming metal concentrating and removing regions.

In the metal-fog-forming metal removing/concentrating apparatus according to the present invention as described above under (2), a mode of embodiment is possible in which the metal-fog-forming metal is Ca and the metal-fog-forming-metal containing molten salt is a Ca-containing molten salt and, further, a mode of embodiment is possible in which the metal-fog-forming metal is Ca and the metal-fog-forming metal containing molten salt is CaCl₂, in the apparatus as described above under (2).

The method of removing/concentrating a metal-fog-forming metal in a molten salt according to the present invention makes it possible to remove the metal-fog-forming metal, for example Ca or Na, that is dissolved in one portion of a molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts therefrom and transfer the same to the other portion of molten salt mixture to increase the concentration of the metal-fog-forming metal therein, both portions of molten salt mixture being respectively in contact with a metal-fog-forming metal containing alloy. This method can be carried out easily and properly using the apparatus according to the present invention.

2. Process for Producing Ti or a Ti Alloy which Includes a Ca Recovery Step, and Apparatus Therefor

For producing Ti or a Ti alloy in stable operation on a commercial scale, it is important to cause the reduction reaction of TiCl₄ or other metal chlorides and the formation of Ca by electrolysis of the molten salt to proceed efficiently and, for stabilizing the operation simultaneously, it is important to increase the concentration of Ca in a CaCl₂-containing molten salt to be fed to the reaction vessel for reducing TiCl₄, to retard concentration fluctuations, and to remove (recover) Ca in the molten salt to be discharged out of the reaction vessel and to be introduced into the electrolytic cell. Further, for enabling Ti production on a commercial scale, it is necessary to increase the rate of feeding of Ca to the reaction vessel (in another word, to continuously treat a large amount of a CaCl₂-containing molten salt in the electrolysis step).

If the Ca concentration in the molten salt fed to the reaction vessel is too low, an unreacted TiCl₄ gas will be discharged out of the vessel. Furthermore, gases of such titanium subchlorides as TiCl₃ and TiCl₂ may be formed and dissolved in the molten salt to react with the Ca formed by electrolysis in the electrolytic cell to form Ti, which will in turn precipitate out on the negative electrode surface and possibly cause operational troubles. Further, it is feared, among others, that TiC, which causes contamination of Ti by C, may be generated.

On the other hand, if the Ca concentration in the molten salt is excessively high, a large amount of Ca may be contained in the molten salt extracted from the reaction vessel; that Ca will evaporate in the separation step, causing a loss.

Further, when the molten salt after separation of Ti in the separation step is returned to the electrolytic cell, a so-called back reaction, namely the reaction of the Ca in the molten salt with the chlorine formed by electrolysis, may occur and lower the current efficiency; the higher the Ca concentration in the molten salt is, the more significant the decrease in current efficiency is. Furthermore, the temperature uniformity in the molten salt (salt bath) within the electrolytic cell will be disturbed by the heat of reaction as resulting from the back reaction, possibly causing troubles in salt bath temperature control.

Therefore, the present inventors made various investigations in an attempt to inhibit the Ca concentration in the molten salt to be fed to the reaction vessel from fluctuating, to maintain that concentration at high levels, and further to suppress the back reaction by rapidly recovering Ca, namely removing Ca, in the molten salt to be fed to the electrolytic cell.

As a result, it was found that it is possible to cause the Ca dissolved in the molten salt to be rapidly absorbed into the molten alloy and to be recovered by keeping the molten salt to be fed to the electrolytic cell in contact with a Ca-containing molten alloy (molten Mg—Ca alloy) and applying a voltage below the decomposition voltage for CaCl₂ so that the electrode bar on the molten alloy side may serve as a negative (−) electrode and the electrode bar on the molten salt side as a positive (+) electrode. This means makes it possible to lower the Ca concentration in the molten salt fed to the electrolytic cell and to suppress the back reaction on the occasion of electrolysis of the molten salt, thereby allowing the formation of Ca efficiently.

On the other hand, it was found that it is effective, for inhibiting the fluctuation of and maintaining at high levels the Ca concentration in the molten salt to be fed to the reaction vessel, to dispose a regulating vessel provided with a Ca supply source between the electrolytic cell and reaction vessel, and to introduce the molten salt into the regulating vessel to render the Ca concentration constant, the molten salt being increased in Ca concentration owing to the formation of Ca by electrolysis, and thereafter to use the same for reduction. By doing so, it becomes possible to maintain the Ca concentration in the molten salt always at a constant high level and allow the reduction reaction to proceed efficiently. It was also revealed that the molten alloy increased in Ca concentration as a result of absorption of Ca therein by voltage application to the molten salt can be used as a Ca supply source for the regulating vessel.

Furthermore, the present inventors made detailed investigations concerning the configuration of an electrolytic cell vessel of a main electrolytic cell, the configuration of electrodes, electrolysis conditions, and a distance between the electrodes, among others, and, as a result, found that when the molten salt is electrolyzed while causing the same to flow in one direction in the vicinity of the cathode surface and the molten salt enhanced in Ca concentration is recovered on the outlet side of the electrolytic cell, it is possible to suppress the back reaction and maintain the current efficiency at a high level and, at the same time, effectively take out the Ca-enriched molten salt alone and, further, continuously treat the CaCl₂-containing molten salt in large quantity and increase the feeding rate of Ca to the reaction vessel.

The Ti or Ti alloy production process including the Ca recovery step according to the present invention and the apparatus therefor have been developed based on the above findings and have constitutions/features as shown in (3) and (4), respectively.

(3) A process for producing Ti or a Ti alloy, comprising: a reduction step in which a CaCl₂-containing molten salt with Ca dissolved therein is held in a reaction vessel and the Ca in the molten salt is allowed to react with a TiCl₄-based metal chloride to form Ti particles or Ti alloy particles in said molten salt; a separation step in which said Ti particles or Ti alloy particles are separated from the molten salt within said reaction vessel or outside the reaction vessel; an electrolysis step in which the molten salt taken out of said reaction vessel is electrolyzed to form Ca to thereby increase the Ca concentration in the molten salt; a return step in which the Ca formed by said electrolysis is introduced, either alone or together with the molten salt, into said reaction vessel; and a Ca recovery step in which while the molten salt separated in said separation step and to be sent to said electrolysis step is kept in contact with a Ca- and Mg-containing molten alloy, a voltage below the decomposition voltage for CaCl₂ is applied so that the electrode bar on the molten alloy side may serve as a negative (−) electrode and the electrode bar on the molten salt side as a positive (+) electrode to thereby cause the Ca dissolved in the molten salt to be absorbed into the molten alloy, and the molten salt decreased in Ca concentration is sent to the electrolysis step.

The “CaCl₂-containing molten salt” so referred to herein is either molten CaCl₂ alone or a molten salt mixture consisted of a molten CaCl₂ and CaF₂ etc to be added for lowering a melting point and adjusting viscosity thereof, among others. It is sometimes referred to as a “molten salt” or a “molten CaCl₂” for short.

The “TiCl₄-based metal chloride” refers to TiCl₂ alone or a mixture of TiCl₄ and other metal chlorides, the metals as constituent being intended for alloy elements in a Ti alloy, for example V, Al and/or Cr. Since the other metal chlorides are also reduced by Ca simultaneously with the reduction of TiCl₄, a Ti alloy can be produced by using such a TiCl₄-based mixed metal chloride as raw material.

When, in the Ti or Ti alloy production process described above under (3), the applied voltage is lower than 3.2 V, it is possible to control its applied voltage at specific numerical value levels and thereby cause the Ca dissolved in CaCl₂ to be rapidly absorbed into the molten alloy without allowing the decomposition of CaCl₂.

When, in the Ti or Ti alloy production process described above under (3), the molten salt increased in Ca concentration in an electrolysis step is introduced into a regulating vessel provided with a Ca supply source and the molten salt is thus brought into contact with the Ca supply source and thereby rendered constant in Ca concentration and thereafter sent to the reduction step, it becomes possible to maintain the Ca concentration in the molten salt introduced into the reaction vessel always at a constant high level to thereby allow the reduction reaction to proceed efficiently.

When, in the production process described above under (3), the molten alloy increased in Ca concentration as a result of absorption of Ca in the Ca recovery step is used as the Ca supply source or part of it for the regulating vessel, it becomes possible to effectively utilize the portion of Ca that has been removed for suppressing the back reaction, which is desirable.

(4) An apparatus for producing Ti or a Ti alloy, comprising: a reaction vessel for holding a CaCl₂-containing molten salt with Ca being dissolved therein and reacting a metal chloride containing TiCl₄, the TiCl₄ being to be fed into said molten salt, with the Ca to cause the formation of Ti particles or Ti alloy particles therein; separation means for separating the Ti particles or Ti alloy particles formed in said molten salt therefrom; an electrolytic cell which holds a left molten salt after separation of said Ti particles or Ti alloy particles and is equipped with an anode and a cathode for carrying out electrolysis in the left molten salt to form Ca on the cathode side; return means for introducing the Ca formed by said electrolysis, either alone or together with an electrolyte molten salt, into the reaction vessel; and Ca recovery means for applying, while keeping the left molten salt separated by the separation means and to be fed to the electrolytic cell in contact with a Ca- and Mg-containing molten alloy, a voltage below the decomposition voltage for CaCl₂ so that the electrode bar on the molten alloy side may serve as a negative (−) electrode and the electrode bar on the molten salt side as a positive (+) electrode to thereby cause the Ca dissolved in the molten salt to be absorbed into the molten alloy, thus decreasing in Ca concentration thereof, and for sending the treated molten salt having a lowered Ca concentration to the electrolytic cell.

When the Ti or Ti alloy production apparatus described above under (4) further comprises a regulating vessel provided with a Ca supply source and intended for introducing molten salt in an electrolytic cell thereinto and bringing the same into contact with the Ca supply source to thereby render the Ca concentration in the molten salt constant and, thereafter, feeding that molten salt into the reaction vessel, the apparatus can be suitably used for carrying out the production process described above under (3).

In accordance with the Ti or Ti alloy production process comprising the Ca recovery step of the present invention, it is possible to rapidly recover the Ca dissolved in molten salt and thus suppress back reaction on the occasion of electrolysis of the molten salt to thereby enhance the efficiency of Ca formation. Furthermore, the process can contribute not only to increase the Ca concentration in the molten salt to be sent to the reduction step, simultaneously with the Ca recovery, and to enhance the efficiency of Ca formation, but also to enhance the efficiency of the TiCl₄ reduction reaction.

Furthermore, the process makes it possible to inhibit the fluctuation of and to maintain at a high level the Ca concentration in the molten salt to be fed to the reaction vessel by using the regulating vessel provided with a Ca supply source and thus carry out the TiCl₄ reduction reaction efficiently and, further, continuously treat the CaCl₂-containing molten salt in large quantity in the electrolysis step and thereby increase the feeding rate of Ca to the reaction vessel.

3. Process for Producing Ti or a Ti Alloy which Includes a Ca Removing/Concentrating Step, and Apparatus Therefor

Furthermore, the present inventors made various investigations to inhibit the fluctuation of and maintain at a high level the Ca concentration in the molten salt to be fed into the reaction vessel, at the same time, to rapidly recover and remove the Ca in the molten salt to be fed to the electrolytic cell therefrom to thereby suppress the back reaction so that the operation can be carried out in an efficient and stable manner on a commercial scale.

As a result, it was revealed that when, in view of the fact that upon application of a voltage below the decomposition voltage for CaCl₂ so that the electrode bar on the molten alloy side may serve as a positive (+) electrode and the electrode bar on the molten salt side as a negative (−) electrode, Ca is transferred from the molten alloy side to the molten salt side, the molten alloy is incorporated into an integrated structure as a common structural element and, while the molten salt is kept in contact with this molten alloy and a voltage (voltage below the decomposition voltage for CaCl₂) is applied so that the electrode bar on the side of first portion of the molten salt may serve as a positive (+) electrode and the electrode bar on the side of second portion of the molten salt being as a negative (−) electrode, it becomes possible to simultaneously effect the absorption of the Ca dissolved in that one molten salt side into the molten alloy and the transfer of Ca from the molten alloy side to the other molten salt side.

Namely, it was revealed that the removal of the Ca dissolved in first portion of the molten salt and the increase in the concentration of Ca dissolved in second portion of the molten salt can be simultaneously and rapidly realized by applying a predetermined voltage.

On the other hand, it was found that, for inhibiting the fluctuation of and maintaining at a high level the Ca concentration in the molten salt to be fed to the reaction vessel, it is effective to dispose, between the electrolytic cell and reaction vessel, a regulating vessel provided with a Ca supply source and introduce the molten salt increased in Ca concentration owing to the formation of Ca by electrolysis into the regulating vessel to thereby render the Ca concentration thereof constant and, thereafter, use the same for reduction.

By doing so, it becomes possible to always maintain the Ca concentration in the molten salt at a constant high level and allow the reduction reaction to proceed efficiently. It was also found that the molten alloy increased in Ca concentration as a result of Ca absorption caused by voltage application to the molten salt can be used as the Ca supply source for the regulating vessel.

Furthermore, the present inventors found that by electrolyzing the molten salt while causing the same to flow in one direction in the vicinity of the cathode surface and by recovering the molten salt increased in Ca concentration on the outlet side of the electrolytic cell, it becomes possible to suppress the back reaction and maintain the current efficiency at a high level and, at the same time, it becomes possible to effectively take out the Ca-enriched molten salt alone and, further, continuously treat the CaCl₂-containing molten salt in large quantity and thereby increase the feeding rate of Ca to the reaction vessel.

The Ti or Ti alloy production process comprising the Ca removing/concentrating step according to the present invention and the apparatus therefor have been developed based on such findings and have constitutions/features shown in (5) and (6) below.

(5) A process for producing Ti or a Ti alloy, comprising: a reduction step in which a CaCl₂-containing molten salt with Ca dissolved therein is held in a reaction vessel and the Ca in the molten salt is allowed to react with a TiCl₄-based metal chloride to form Ti particles or Ti alloy particles in said molten salt; a separation step in which said Ti particles or Ti alloy particles are separated from the molten salt inside or outside said reaction vessel; an electrolysis step in which the molten salt taken out of said reaction vessel is electrolyzed to form Ca, thereby increasing the Ca concentration in the molten salt; a return step in which the Ca formed by said electrolysis is introduced, either alone or together with the molten salt, into said reaction vessel; and a Ca removing/concentrating step in which a voltage below the decomposition voltage for CaCl₂ is applied so that the electrode plate on the molten salt side within a Ca-removing region serving to hold the molten salt separated in said separation step and to be sent to said electrolysis step may serve as a positive (+) electrode against the electrode plate on the molten salt side within a Ca-concentrating region, apart from the Ca-removing region, serving to hold the molten salt to be sent to said reduction step, and the molten salt reduced in Ca concentration in the Ca-removing region is sent to the electrolysis step and the molten salt increased in Ca concentration in the Ca-concentrating region is sent to the reduction step.

The terms “CaCl₂-containing molten salt” and “TiCl₄-based metal chloride” are as exactly as defined in the above “2: Ti or Ti alloy production process comprising a Ca recovery step”.

When, in the Ti or Ti alloy production process described above under (5), the applied voltage is lower than 3.2 V, it is possible to control the applied voltage at a specific numerical value level and cause rapid absorption of the Ca dissolved in CaCl₂ into the molten alloy without allowing decomposition of CaCl₂. Further, when the molten salt increased in Ca concentration in the electrolysis step is introduced into the regulating vessel provided with a Ca supply source and brought into contact with the Ca supply source to render the Ca concentration in the molten salt constant and, thereafter, the resulting molten salt is sent to the reduction step, it enables the Ca concentration in the molten salt to be introduced into the reaction vessel to be maintained always at a constant high level, thus allowing the reduction reaction to proceed efficiently.

(6) An apparatus for producing Ti or a Ti alloy, comprising: a reaction vessel for holding a CaCl₂-containing molten salt with Ca dissolved therein and reacting a TiCl₄-based metal chloride fed into the molten salt with said Ca to cause the formation of Ti particles or Ti alloy particles therein; separation means for separating the Ti particles or Ti alloy particles formed in said molten salt from the molten salt; an electrolytic cell which holds the molten salt after separation of said Ti particles or Ti alloy particles therefrom and is equipped with an anode and a cathode for carrying out electrolysis in said molten salt to form Ca on the cathode side; are turn means for introducing the Ca formed by said electrolysis, either alone or together with a treated molten salt, into said reaction vessel; and a Ca removing/concentrating unit which comprises: (a) a Ca-removing region for holding the molten salt separated in said separation step and to be fed to said electrolysis step; and (b) a Ca-concentrating region, apart from the Ca-removing region, serving to hold the molten salt to be sent to said reduction step and which serves to apply a voltage below a decomposition voltage for CaCl₂ so that the electrode plate on the molten salt side in the Ca-removing region may serve as a positive (+) electrode against the electrode plate on the molten salt side in the Ca-concentrating region and, further, serves to send the molten salt thus decreased in Ca concentration in the Ca-removing region to the electrolysis step and the molten salt thus increased in Ca concentration in the Ca-concentrating region to the reduction step.

When the Ti or Ti alloy production apparatus described above under (6) further comprises a regulating vessel provided with a Ca supply source and intended for introducing molten salt in the electrolytic cell thereinto and bringing the same into contact with the Ca supply source to thereby render the Ca concentration in the molten salt constant and, thereafter, feeding that molten salt into the reaction vessel, the apparatus can be suitably used for carrying out the production process described above under (5).

In accordance with the Ti or Ti alloy production process comprising a Ca removing/concentrating step according to the present invention, it is possible to rapidly remove the Ca dissolved in the molten salt therefrom and thus suppress the back reaction on the occasion of electrolysis of the molten salt to thereby enhance the efficiency of Ca formation. Furthermore, the process can contribute not only to increase the Ca concentration in the molten salt to be sent to the reduction step, simultaneously with the Ca removal, and enhance the efficiency of Ca formation but also to enhance the efficiency of the TiCl₄ reduction reaction.

Furthermore, the process makes it possible to inhibit the fluctuation of and maintain at a high level the Ca concentration in the molten salt to be fed to the reaction vessel by using the regulating vessel provided with a Ca supply source and thus carry out the TiCl₄ reduction reaction efficiently and, further, continuously treat the CaCl₂-containing molten salt in large quantity in the electrolysis step to thereby increase the feeding rate of Ca to the reaction vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an example of the principal constitution/feature of an apparatus to be used in carrying out the method of removing/concentrating a metal-fog-forming metal in a molten salt according to the present invention.

FIG. 2 is a schematic representation of the relation between the voltage applied between molten Mg—Ca alloy and molten CaCl₂ and the current flowing between both electrodes. FIG. 2 (a) is a representation for the case where Ca is not dissolved in CaCl₂ (before addition of Ca), and FIG. 2 (b) is a representation for the case where Ca is dissolved in CaCl₂ (after addition of Ca).

FIG. 3 is a schematic representation of an example of the principal constitution/feature of an apparatus to be used in carrying out the method of recovering (i.e. method of removing) a metal-fog-forming metal in a molten salt according to the present invention.

FIG. 4 is a schematic representation of an example of the constitution/feature of an apparatus to be used in carrying out the Ti or Ti alloy production process including a Ca recovery step according to the present invention.

FIG. 5 is a schematic representation of another example of the constitution/feature of an apparatus to be used in carrying out the Ti or Ti alloy production process including a Ca recovery step according to the present invention.

FIG. 6 is a schematic representation of the constitution/feature of an apparatus to be used in carrying out the Ti or Ti alloy production process including a Ca removing/concentrating step according to the present invention.

FIG. 7 is a schematic representation of another example of the constitution/feature of an apparatus to be used in carrying out the Ti or Ti alloy production process including a Ca removing/concentrating step according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, the subject matters of the following three aspects of the present invention are described in more detail: “1: method of removing/concentrating a metal-fog-forming metal in a molten salt, and apparatus therefor”, “2: process for producing Ti or a Ti alloy which includes a Ca recovery step, and apparatus therefor” and “3: process for producing Ti or a Ti alloy which includes a Ca removing/concentrating step, and apparatus therefor”. The case where a metal-fog-forming metal is Ca is exemplified in the following.

1. Method of Removing/Concentrating a Metal-Fog-Forming Metal in a Molten Salt, and Apparatus Therefor

FIG. 1 is a schematic representation of an example of the principal constitution/feature of an apparatus to be used in carrying out the method of removing/concentrating a metal-fog-forming metal in molten salt according to the present invention. The constitution/feature shown is the same as that of a Ca removing/concentrating unit 28 in FIG. 6 or FIG. 7, which is to be hereinafter referred to and in which the same reference numerals are used. Further, in FIG. 1, Ca denotes a metal-fog-forming metal, a molten CaCl₂ denotes a molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts, and a molten Mg—Ca alloy denotes a metal-fog-forming metal containing molten alloy, and these designations are used in the description which follows.

As shown in FIG. 1, this apparatus has a Ca removing/concentration vessel 28 a, and a molten CaCl₂ is held in this vessel 28 a in a condition separated into a Ca-concentrating region 29 and a Ca-removing region 30 by a partition wall 31, and a molten Mg—Ca alloy 8 is held thereon in contact with each portion of the molten salt held in the Ca-concentrating region 29 and Ca-removing region 30.

Further, on the bottom of the Ca removing/concentrating vessel 28 a, there is disposed an electrode plate 33 for applying a voltage below a decomposition voltage for CaCl₂ so that the molten salt side in the Ca-removing region 30 may be on the positive (+) electrode side against the molten salt side in the Ca-concentrating region 29. Generally, an electrode plate 34 is disposed so that the electrode plate on the molten salt side in the Ca-concentrating region 29 may serve as a negative (−) electrode, as shown in the figure. In the example shown, the Ca-concentrating region 29 and Ca-removing region 30 are separated from each other by the partition wall 31 but the invention is not limited to this constitution/feature. Thus, for example, a constitution/feature such that both regions are apart from each other by the use of two independent, detachable cells may also be employed.

In carrying out the method of removing/concentrating Ca, the Ca being a metal-fog-forming metal, in a molten salt using the apparatus shown in FIG. 1, a molten CaCl₂ as molten salt with Ca dissolved therein is first held in the Ca-concentrating region 29 in the Ca removing/concentrating vessel 28 a as well as in the Ca-removing region 30 separated from the concentrating region 29 by the partition wall 31. Further, a molten Mg—Ca alloy 8 is held onto the molten salt in both regions 29 and 30 in contact with both portions of the molten salt.

Then, a voltage below the decomposition voltage for CaCl₂ is applied so that the electrode plate 33 on the molten salt side in the Ca-removing region 30 may serve as a positive (+) electrode against the electrode plate 34 on the molten salt side in the Ca-concentrating region 29.

Upon this voltage application, the molten Mg—Ca alloy 8 existing in the adjacent surface of contact with the molten salt in the Ca-removing region 30 functions as a negative (−) electrode relative to the molten salt side (+electrode side) in the Ca-removing region 30 and, therefore, the dissolved Ca transfers to the molten Mg—Ca alloy 8 side and is absorbed thereinto, as shown by the arrows in FIG. 1. As a result, the dissolved Ca in the Ca-removing region 30 is removed therefrom and the Ca concentration in the Mg—Ca alloy 8 increases.

On the other hand, the molten Mg—Ca alloy in the adjacent surface of contact with the molten salt in the Ca-concentrating region 29 functions as a positive (+) electrode relative to the molten salt side (−electrode side) in the Ca-concentrating region 29. Therefore, the Ca in the molten Mg—Ca alloy 8 transfers to the molten salt side in the Ca-concentrating region 29 and the Ca concentration in the Ca-concentrating region 29 rises, namely that concentration is increased.

In this manner, by applying a voltage below the decomposition voltage for CaCl₂ so that the electrode plate on the molten salt side in the Ca-removing region 30 may serve as a positive (+) electrode against the electrode plate on the molten salt side in the Ca-concentrating region 29, it becomes possible to remove the dissolved Ca in the Ca-removing region 30 therefrom and at the same time increase the dissolved Ca concentration in the Ca-concentrating region 29. In addition, when an apparatus having the main structural elements shown in FIG. 1 is used, these simultaneous treatments can be carried out with ease using a very facile apparatus in terms of both the configuration and constitution/feature.

That the voltage to be applied should be lower than the decomposition voltage for CaCl₂ is to avoid the formation of Ca in case of the decomposition of CaCl₂.

As for the electrodes for the voltage application mentioned above, it is recommended that iron or like metals be used as the negative (−) electrode and a graphite electrode or like insoluble electrodes as the positive (+) electrode.

In a practical mode of operation, it is possible to control the operation while judging the degrees of the above-mentioned removal and concentration of the dissolved Ca, based on the “limiting current” which is observable as a result of such Ca transfer from the molten salt side (+electrode side) to the molten alloy side (−electrode side), as mentioned below, and which can be regarded as the measure (indicator).

FIG. 2 is a schematic representation of the relation between applied voltage and flowing current, the voltage being applied between molten Mg—Ca alloy and molten CaCl₂, the current flowing between both electrodes, which is obtained based on the results of an experimental investigation made by the present inventors. FIG. 2 (a) is for the case where Ca is not dissolved in CaCl₂ (before addition of Ca), and FIG. 2 (b) is for the case where Ca is dissolved therein (after addition of Ca).

When Ca is not dissolved, no current flows at all even if increasing the applied voltage, as shown (cf. FIG. 2 (a)). On the contrary, after the addition of Ca, a benign electric current begins to flow just when a slight voltage is applied and, thereafter, upon increasing the voltage, an almost constant current flows until the applied voltage comes close to the decomposition voltage Vb (3.2 V) for CaCl₂ (such current is called “limiting current”). When the voltage is further increased, CaCl₂ is electrolyzed and therefore the current value rapidly increases (FIG. 2 (b)).

The limiting current mentioned above is observed as a result of the transfer of Ca from the molten salt side (+(positive) electrode side) to the molten alloy side (−(negative) electrode side) (i.e. absorption of the Ca, dissolved in CaCl₂, into the molten alloy), and the intensity thereof depends on the concentration of Ca dissolved in CaCl₂ and the limiting current value decreases with the decrease in Ca concentration. According to the investigation results obtained by the present inventor's, the Ca concentration was about 0.01% by mass when the limiting current value was 0.14 A/cm².

Therefore, it is also possible, for example, to carry out the operation in such a manner that the voltage to be applied be set at a level below the decomposition voltage for CaCl₂ and the treatment be finished at the time that the limiting current value, which gradually decreases, reaches a predetermined current density, although the mode of operation may vary depending on the intended use of the molten salt obtained by the method of removing/concentration a metal-fog-forming metal in a molten salt as mentioned above.

Since the limiting current value decreases as the Ca concentration in the molten salt decreases, as mentioned above, it is desirable, for rapidly decreasing the Ca concentration and thereby enhancing the Ca removal efficiency, that the contact area between molten salt and molten alloy in the Ca-removing region be broadened. Since the same amount of electric current as that flowing in the Ca-removing region flows also in the Ca-concentrating region, it is desirable that the above-mentioned contact area in the Ca-concentrating region be also widened in the same manner as above to lower the resistance.

The case where “the metal-fog-forming metal is Ca and the metal-fog-forming metal containing molten salt is a Ca-containing molten salt” and, further, the case where “the metal-fog-forming metal is Ca and the metal-fog-forming metal containing molten salt is CaCl₂” are desirable modes of embodiment of the process according to the present invention.

In either of these modes of embodiment, the metal-fog-forming metal is limited to Ca because the production of metallic Ti by Ca reduction of TiCl₄ through an intermediary molten salt is thought to be one of the promising fields from the viewpoint of metal-fog-forming metal utilization; further, the molten salt can also be restricted to CaCl₂ which is relatively inexpensive and easy to handle.

In the process according to the present invention, Ca can be removed while “the applied voltage is maintained at a level lower than 3.2 V (namely, voltage below the decomposition voltage for CaCl₂)”. By this measure, it becomes possible to control the applied voltage at specific numerical value levels and cause Ca to be rapidly absorbed into the molten alloy by making a potential difference between the electrode plate on the molten salt side and the electrode plate on the molten alloy side, without allowing decomposition of CaCl₂.

On that occasion, even when the applied voltage is slight, the Ca removing effect can be retained and, therefore, the lower limit thereto is not particularly specified. For effective Ca removal, however, it is preferable that the applied voltage be not lower than 0.01 V.

The apparatus for removing/concentrating a metal-fog-forming metal in a molten salt according to the present invention has such main structural elements as shown in FIG. 1 as above and makes it possible to carry out the above-mentioned method of removing/concentrating a metal-fog-forming metal according to the present invention in an easy and proper manner.

FIG. 3 is a schematic representation of an example of the principal constitution/feature of an apparatus to be used in carrying out the method of recovering (i.e. method of removing) a metal-fog-forming metal in molten salt according to the present invention. The constitution/feature shown in FIG. 3 is the same as that of Ca recovery means 5 shown in FIG. 4 and FIG. 5, which is to be hereinafter referred to and in which the same reference numerals are used. In FIG. 3, like in the above-mentioned case shown in FIG. 1, the metal-fog-forming metal is represented by Ca, so is the molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts by a molten CaCl₂, and the metal-fog-forming metal containing molten alloy by a molten Mg—Ca alloy, and these designations are used in the description which follows.

As shown in FIG. 3, the Ca recovery means 5 comprises a Ca recovery vessel 6 and, within this recovery vessel 6, there is held a molten CaCl₂ 7 and, thereupon, a molten Mg—Ca alloy is held in contact with the molten salt 7. An electrode bar inserted in the molten salt 7 constitutes a positive (+) electrode and an electrode bar 10 inserted in the molten Mg—Ca alloy 8 constitutes a negative (−) electrode.

In recovering the metal-fog-forming metal Ca using the Ca recovery means 5 shown in FIG. 3, a molten CaCl₂ with Ca dissolved therein is first held in a Ca recovery vessel 6. Further, the molten Mg—Ca alloy 8 is held onto the thus-held molten salt 7 to be in contact with the molten salt 7.

Then, a voltage below the decomposition voltage for CaCl₂ is applied so that the electrode bar 10 inserted in the molten Mg—Ca alloy 8 may serve as a negative (−) electrode and the electrode bar 9 inserted in the molten salt 7 as a positive (+) electrode. By this voltage application, that portion of the molten Mg—Ca alloy 8 present in the adjacent surface of contact with the molten salt 7 within the Ca recovery vessel 6 functions as a negative (−) electrode relative to the molten salt side (+electrode side) within the Ca-removing vessel 6 and, therefore, the Ca dissolved therein transfers to the molten Mg—Ca alloy 8 side, as indicated by the arrows in FIG. 3, and is absorbed thereinto. As a result, the dissolved Ca in the Ca-removing vessel 6 is recovered (removed).

An application example of the method of removing/concentrating a metal-fog-forming metal according to the present invention as shown hereinabove in FIG. 1 is described below under “3: Ti or Ti alloy production process including a Ca removing/concentrating step, and apparatus therefor”. An application example of the method of removing a metal-fog-forming metal according to the present invention as shown hereinabove in FIG. 3 is described below under “2: Ti or Ti alloy production process including a Ca recovery step, and apparatus therefor”.

2. Ti or Ti Alloy Production Process Including a Ca Recovery Step, and Apparatus Therefor

FIG. 4 is a schematic representation of an example of the constitution/feature of an apparatus to be used in carrying out the Ti or Ti alloy production process including a Ca recovery step according to the present invention. The figure shows the case where TiCl₄ alone is used as the raw material.

As shown in FIG. 4, this apparatus comprises: a reaction vessel 1 for holding a CaCl₂-containing molten salt with Ca dissolved therein and for reacting TiCl₄ fed into said molten salt with Ca to form Ti particles; a separation means 2 for separating said Ti particles formed in the molten salt therefrom; an electrolytic cell 3 for electrolyzing the molten salt after separation of said Ti particles therefrom to form Ca on the cathode side; a return means 4 for introducing the Ca formed upon electrolysis into the reaction vessel 1; and a Ca recovery means 5 for removing the Ca dissolved in the molten salt separated in the separation means and to be fed to the electrolytic cell.

The Ca recovery means 5 shown by way of example in FIG. 4 comprises its essential constituents as shown, and the molten salt 7 separated in the above-mentioned separation means 2 is introduced into the Ca recovery vessel 6. Thereon is held a Ca- and Mg-containing molten alloy 8 (also referred to as “molten Mg—Ca alloy” or as “molten alloy” for short). The electrode bar 9 inserted in the molten salt 7 constitutes a positive (+) electrode and the electrode bar 10 inserted in the molten Mg—Ca alloy 8 constitutes a negative (−) electrode.

The electrolytic cell 3 comprises a piping-like (cylindrical) electrolytic cell vessel 3 a elongated in one direction and intended for holding a CaCl₂-containing molten salt, and likewise, cylindrical anode 11 and a round column-shaped cathode 12, each disposed within the electrolytic cell vessel 3 a along the length-wise direction of the vessel 3 a. One end (bottom plate 13), in a lengthwise direction, of the electrolytic cell vessel 3 a is equipped with a molten salt supply port 14, and the other end (cover plate 15) is equipped with a molten salt extraction port 16. The surface of the anode 11 and the surface of the cathode 12 are disposed substantially vertically in a facing relation with each other and, further, a partition wall 17 is disposed between the anode 11 and cathode 12 so as to inhibit the Ca formed by electrolysis of the molten salt from passing therethrough. A cooling device 18 is provided surrounding the outside surface of the anode 11.

In the apparatus shown in FIG. 4, a decanter type centrifuge (high-temperature decanter) 19 and a separating vessel 20 are used as a separation means 2.

In carrying out a Ti or Ti alloy production process according to the present invention using the apparatus shown in FIG. 4, the molten salt fed from the electrolytic cell 3 via a return means 4 is first held in a reaction vessel 1 and the TiCl₄ fed through a TiCl₄ supply port 21 is caused to react with the Ca in the molten salt to form Ti particles in the molten salt. Thus, the “reduction step” is carried out.

In this reduction step, the molten salt held in the reaction vessel 1 is not at rest but is gradually moving downward from the upper part of the reaction vessel 1 toward the bottom and, during the downward movement, TiCl₄ as raw material is reduced by the Ca in the molten salt to form Ti particles. In cases where a mixed metal chloride comprising TiCl₄ and at least one of other metal chlorides (e.g. chloride of V, Al, Cr, etc.) is used as the raw material, the other metal chlorides are also reduced by Ca, as mentioned above, and, therefore, by preliminarily adding predetermined amounts of such metal chlorides to TiCl₄, it becomes possible to form Ti alloy particles and finally produce a Ti alloy.

The Ti particles formed in the reduction step are separated from the molten salt in the “separation step”.

When an appropriate reaction vessel is used, the separation of the Ti particles from the molten salt can be realized also within the reaction vessel but, in this case, the process is carried out batch-wise. Therefore, for enhancing productivity, it is recommended, for example, that the molten salt with Ca dissolved therein be continuously fed using a reaction vessel of the type shown in FIG. 4 and the formed Ti particles be extracted out of the reaction vessel and separated from the molten salt outside the vessel.

In the separation step, when the apparatus shown in FIG. 4 is used, the Ti particles are first separated and recovered in the high-temperature decanter 19 and then the molten salt adhering to the Ti particles is removed in the separating vessel 20.

The decanter type centrifuge is a centrifuge of the type such that a suspended substance is caused to settle by centrifugation by rotating a rotary cylinder at a high speed and enables high-speed treatment and has high dehydration performance. A type allowing high-temperature treatment has also already been developed and can be used as the high-temperature decanter 19 in this separation step.

The Ti particles taken out of the high-temperature decanter 19 are heated and melted by plasma emitted from a plasma torch 22 in the separation vessel 20, and the melt is cast into a mold 23 to give a Ti ingot 24.

On the other hand, the molten salt separated from Ti particles (such molten salt herein after referred to as “adherent molten salt”) may possibly contain Ti particles. Therefore, returning of this adherent molten salt to the electrolysis step may possibly cause problems; it is therefore desirable that it be returned to the reaction vessel 1, as indicated in FIG. 4. In addition, a certain amount of Ca remains in the adherent molten salt, so that it is reasonable to return it to the reaction vessel 1 from the viewpoint of effective Ca utilization as well.

The molten salt reduced in Ca concentration as separated in said high-temperature decanter 19 is sent to the “Ca recovery step”. Namely, said molten salt is introduced into the Ca recovery vessel 6 and kept in contact with the molten Mg—Ca alloy 8 and a voltage is applied so that the electrode bar on the molten alloy side may serve as a negative (−) electrode and the electrode bar on the molten salt side as a positive (+) electrode. The applied voltage on that occasion is lower than the decomposition voltage for CaCl₂. It becomes possible thereby to cause the Ca dissolved in CaCl₂ to be rapidly absorbed into the molten alloy without allowing decomposition of CaCl₂ and rapidly send the molten salt decreased in Ca concentration to the electrolysis step. Since the Ca concentration in the molten salt is reasonably lowered, the back reaction is suppressed.

As for the electrodes for the above-mentioned voltage application, it is recommended that iron or a like metal be used as the negative (−) electrode and a graphite electrode or a like insoluble electrode as the positive (+) electrode.

As shown in FIG. 2, the limiting current results from the transfer of Ca from the molten salt side (+electrode side) to the molten alloy side (−electrode side) and the intensity thereof depends on the concentration of Ca dissolved in CaCl₂, and the limiting current decreases as the Ca concentration decreases. According to the investigation results obtained by the present inventors, the Ca concentration was about 0.01% by mass when the limiting current density was 0.14 A/cm².

Since the limiting current becomes small as the Ca concentration in the molten salt decreases, as mentioned above, it is desirable, for rapidly decreasing the Ca concentration and thereby enhancing the Ca removal (recovery) efficiency, to use a large-sized Ca recovery vessel so that the contact area between molten salt 7 and molten Mg—Ca alloy 8 may be broadened.

By removing Ca by selecting “the voltage applied at a level lower than 3.2 V (namely, voltage below the decomposition voltage for CaCl₂)” in the Ti or Ti alloy production process according to the present invention, it becomes possible to control the applied voltage at specific numerical value levels and cause Ca to be rapidly absorbed into the molten alloy by implementing a potential difference between the electrode bar on the molten salt side and the electrode bar on the molten alloy side, without allowing decomposition of CaCl₂. Since, even when the applied voltage is benign, the voltage application produces a Ca-removing effect, the lower limit to the applied voltage is not particularly specified. For effective Ca removal, however, it is desirable that the applied voltage be not lower than 0.01 V.

The molten salt reduced in Ca concentration in the Ca recovery step is sent to the “electrolysis step” and electrolyzed to form Ca, whereupon the Ca concentration in the molten salt is increased.

Thus, as shown in FIG. 4, the molten salt is first fed into and held in the space between the cathode 12 and partition wall 17 in the electrolytic cell 3. Since the electrolytic cell 3 has a shape elongated in one direction (in the example shown, a piping-like (cylindrical) shape elongated in a vertical direction), it is possible to provide the molten salt in the vicinity of the surface of the cathode 12 with a flow rate in one direction and cause the molten salt to flow in one direction in the vicinity of the surface of the cathode 12 by continuously or intermittently feeding the molten salt from one end of the electrolytic cell 3 to the space between the anode 11 and cathode 12. The feeding of the molten salt is generally carried out continuously. Depending on the subsequent step and/or other factors, the feeding may be carried out intermittently, namely the feeding of the molten salt may be temporarily halted and then resumed.

Then, the molten salt is electrolyzed. While the molten salt is allowed to flow in one direction in the vicinity of the surface of the cathode 12, the molten salt is electrolyzed to form Ca on the cathode surface. Since the electrolytic cell 3 has a shape elongated in one direction and, further, in the example shown in FIG. 4, the distance between the anode 11 and cathode 12 is set to be relatively short so that the electrolysis voltage may be suppressed to a low level, it is possible to effectively draw out only the molten salt enriched in Ca while inhibiting the mixing of the Ca-rarified molten salt in the vicinity of the molten salt supply port 14 with the Ca-enriched molten salt in the vicinity of the molten salt extraction port 16 by electrolysis.

While the extraction system employed in the electrolytic cell shown by way of example in FIG. 4 is such that CaCl₂ is fed into the electrolytic cell 3 from below the cell 3 and taken out at the top, a converse system such that CaCl₂ is fed from above the electrolytic cell 3 and extracted at the bottom can also be employed.

In the electric cell used in this process, the anode surface and cathode surface are disposed substantially vertically in a facing relation with each other while the molten salt in the vicinity of the cathode surface is given a flow rate in one direction and, therefore, the direction of flow of the molten salt is vertical and the chlorine gas generated on the anode side readily floats up to the surface and can be recovered with ease.

In carrying out the electrolysis of the molten salt using this electrolytic cell, a large amount of the molten salt is treated continuously, so that it is preferable to effectively carry out heat removal in the electrolytic cell. More specifically, it is desirable, for example, that a cooling device be disposed in the central portion of the cathode for removing the heat of reaction from inside the cathode. A tube-type heat exchanger, for instance, is suited for use as the cooling device.

When a cooling device (heat exchanger) is disposed on the anode side as well, the heat removal efficiency is further enhanced. In the electrolytic cell shown in FIG. 4, the cooling device 18 disposed so as to surround the anode 11 is an example of such cooling device.

The Ca formed upon electrolysis in the electrolysis step is introduced, either alone or together with the molten salt, into the reaction vessel via the “return step”.

When the apparatus shown in FIG. 4 is used, the molten salt increased in Ca concentration in the electrolytic cell is obtained and the Ca is introduced, together with the molten salt, into the reaction vessel via the return step.

When, however, use is made of an electrolytic cell having a constitution/feature such that the Ca formed upon electrolyzing the molten salt can be recovered as such, namely as Ca alone (including, however, the condition such that a slight amount of the molten salt is admixed in the Ca), it is possible to employ, in the Ti or Ti production process according to the present invention, a mode of embodiment such that the Ca formed by electrolysis is introduced, while being dissolved in a molten salt, into the reaction vessel.

Thus, in such mode of embodiment, the molten salt is not utilized as a transfer medium for Ca in the return step but the Ca formed is transferred as such to a site in the vicinity of the reaction vessel and dissolved there in a separately prepared molten salt and then introduced into the reaction vessel; a reduction in transfer cost can be then expected.

It is further possible to employ a mode of embodiment such that when a reaction vessel which makes it possible to feed the Ca thus formed into the reaction vessel for reaction with TiCl₄ is used, the Ca alone is introduced into the reaction vessel.

FIG. 5 is a schematic representation of another example of the constitution/feature of an apparatus to be used in carrying out the Ti or Ti alloy production process including a Ca recovery step according to the present invention.

This apparatus is a modification to the apparatus shown in FIG. 4 that is made by further providing a regulating vessel 25 for introducing the molten salt in the electrolytic cell 3 thereinto and bringing the same into contact with a Ca supply source to render the Ca concentration in the molten salt constant and, thereafter, feeding the resulting molten salt to the reaction vessel 1.

The production process shown in FIG. 5 is a modification of the Ti or Ti alloy production process according to the present invention and includes the step of “introducing the molten salt increased in Ca concentration in the electrolysis step into a regulating vessel provided with a Ca supply source and bringing the molten salt with the Ca supply source to thereby render the Ca concentration in the molten salt constant and feeding the resulting molten salt to the reduction step”.

By using the apparatus shown in FIG. 5, it becomes possible to introduce the Ca-enriched molten salt taken out of the electrolytic cell 3 into the regulating vessel 25 and brining the same into contact with the Ca supply source 26 to render the Ca concentration in the molten salt 27 constant and then feed the resulting molten salt into the reaction vessel 1. The process is thus a modification in which the treatment in the regulating vessel 25 is incorporated in the return step.

The Ca concentration in the molten salt enriched in Ca in the electrolysis step varies with certain changes in electrolysis conditions in the electrolytic cell 3. Therefore, when the molten salt subjected to electrolysis treatment in the electrolytic cell 3 is directly introduced into the reaction vessel 1, the Ca concentration is not always maintained at a constant level and, therefore, the formation of titanium subchlorides and decreases in current efficiency due to the back reaction, among others, may occur, as mentioned herein above, and, in some instances, the TiCl₄ reduction reaction efficiency may be lowered and/or the operation may become difficult to carry out stably.

Therefore, the molten salt increased in Ca concentration by using the electrolytic cell 3 in the electrolysis step is introduced into the regulating vessel 25 provided with a Ca supply source 26 and brought into contact with the Ca supply source 26 and thereby rendered constant in Ca concentration; the resulting molten salt can be used for reducing TiCl₄ in the reduction step.

The flow rate of the adherent molten salt separated from Ti particles in the separation vessel 20 is very low as compared with the flow rate of the molten salt introduced from the electrolytic cell 3 into the reaction vessel 1 via the regulating vessel 25, so that the adherent molten salt may be returned directly to the reaction vessel 1, as mentioned above. It is preferable, however, to once introduce it into the regulating vessel 25 and, after rendering the Ca concentration constant, introduce the same into the reaction vessel 1, as shown in FIG. 3.

Usable as the Ca supply source 26 are molten metallic Ca and molten alloys containing Ca at relatively high content levels, such as molten Mg—Ca alloy.

Thus, molten metallic Ca or a molten Mg—Ca alloy, for instance, is caused to float on the molten salt 27 increased in Ca concentration and introduced into the regulating vessel 25 and such Ca supply source 26 and the molten salt 27 are kept in contact with each other. By doing so, if the Ca concentration in the molten salt 27 is lower than the saturation solubility thereof, Ca is supplied from the Ca supply source 26 to the molten salt 27 and, in this manner, the Ca concentration can be maintained at a level in the vicinity of the saturation solubility.

In case the Ca concentration in the molten salt 27 is at its saturation solubility and the precipitated metallic Ca coexist therein, the metallic Ca floats up to the surface and is separated in the regulating vessel 25 owing to the specific gravity difference and, thus, the Ca concentration can be maintained at a level in the vicinity of the saturation solubility. Furthermore, by controlling the temperature of the molten salt 27 on the occasion of extraction from the regulating vessel 25 to a constant level, it becomes possible to control the Ca concentration at a constant level in the vicinity of the saturation solubility at that temperature.

Therefore, by providing the regulating vessel 25 and introducing thereinto the molten salt taken out of the electrolytic cell 3, irrespective of whether the Ca concentration in the molten salt enriched in Ca in the electrolytic cell 3 is at or below the saturation solubility, it becomes possible to feed the molten salt whose Ca concentration is at a constant level in the vicinity of the saturation solubility thereof to the reaction vessel 1 and allow the TiCl₄ reduction reaction to proceed efficiently and, thus, carry out the operation stably.

If, however, the electrolysis in the electrolytic cell 3 is carried out to an extent such that the Ca concentration exceeds the saturation solubility, metallic Ca may precipitate out within the electrolytic cell 3, possibly causing such a trouble as electrolytic cell obstruction. Therefore, in increasing the Ca concentration in the electrolytic cell 3, it is preferable to carry out the operation in a manner such that the electrolysis is carried out under control so that the Ca concentration may be reasonably high but just short of the saturation solubility, and the molten salt high in Ca concentration but lower than the saturation solubility is introduced into the regulating vessel 25 and brought into contact with the Ca supply source 26 to thereby adjust the Ca concentration to a constant level in the vicinity of the saturation solubility.

The production process according to the present invention is preferably one in which the Ca supply source in the regulating vessel shown in FIG. 5 is specified in manner such that “the molten alloy increased in Ca concentration as a result of absorption of Ca in the Ca recovery step is used as the whole Ca supply source or part of it in the regulating vessel”.

Thus, as shown in FIG. 5, the molten alloy 8 increased in Ca concentration as a result of absorption of Ca in the Ca recovery step (a Ca recovery means 5) is transferred to the regulating vessel 25 for use as the Ca supply source 26. The whole Ca supply source 26 may be covered by the molten alloy transferred from the Ca recovery step or the molten alloy may be used as part of the Ca supply source 26 when insufficient in quantity. In either case, the Ca removed from the molten salt separated in the high-temperature decanter 19 and to be sent to the electrolysis step so as to suppress the back reaction can be utilized efficiently.

The Ti or Ti alloy production apparatus according to the present invention is an apparatus to be used in carrying out the Ti or Ti alloy production process including such a Ca recovery step as mentioned above and the constitution/feature thereof is as schematically shown in FIG. 4. The functions of the respective structural elements are the same as mentioned above and, when this apparatus is used, the Ti or Ti alloy production process (including the mode of embodiment la) according to the present invention can be properly carried out.

3. Process for Producing Ti or a Ti Alloy which Includes a Ca Removing/Concentrating Step, and Apparatus Therefor

FIG. 6 is a schematic representation of the constitution/feature of an apparatus to be used in carrying out the Ti or Ti alloy production process including a Ca removing/concentrating step according to the present invention. Here, too, the case of using TiCl₄ alone as raw material is described.

This apparatus is a modification to the apparatus shown in FIG. 4 that is made by providing a Ca removing/concentrating apparatus instead of the Ca recovery means and changing the transfer route of the molten salt accordingly.

Thus, as shown in FIG. 6, the apparatus comprises: a reaction vessel 1 for holding a CaCl₂-containing molten salt with Ca dissolved therein and allowing TiCl₄ fed into the molten salt to react with the Ca to form Ti particles; a separation means 2 for separating the Ti particles formed in the molten salt from the molten salt; an electrolytic cell 3 for electrolyzing the molten salt after separation of the Ti particles to form Ca on the cathode side; a return means 4 for introducing the Ca formed by electrolysis into the reaction vessel 1; and a Ca removing/concentrating unit 28 for removing the Ca dissolved in the molten salt that is separated by the separation means (high-temperature decanter) and is to be fed to the electrolytic cell 3, and for simultaneously increasing the concentration of Ca dissolved in the molten salt separated by a separation means (a separating vessel) and to be introduced into the reaction vessel 1.

The Ca removing/concentrating unit 28 whose main parts are shown in the figure comprises a Ca removing/concentrating vessel 28 a, wherein a molten CaCl₂ is held in the vessel 28 a in a condition separated into a Ca-concentrating region 29 and a Ca-removing region 30 by a partition wall 31 and, thereon, a molten Mg—Ca alloy 8 is held in contact with either portion of the molten salt that are present in the Ca-concentrating region 29 and Ca-removing region 30.

Further, at the bottom of the Ca-removing region 30, there is provided an electrode plate 33 for applying a voltage below the decomposition voltage for CaCl₂ so that it may serve as a positive (+) electrode against an electrode plate 34 on the molten salt side in the Ca-concentrating region 29. Although, in the example shown, the Ca-concentrating region 29 and Ca-removing region 30 are separated from each other by the partition wall 31, the constitution/feature thereof is not limited to this. Thus, for example, both regions may be separated from each other by use of two independent, detachable vessels.

In carrying out the Ti or Ti alloy production process described above under (2) using the apparatus shown in FIG. 6, the procedures in the “reduction step”, “separation step”, “electrolysis step” and “return step” are fundamentally the same as in the above-mentioned case of using the apparatus shown in FIG. 4. In the same manner as in that case, a Ti alloy can be produced as a final product when TiCl₄-based mixed metal chlorides are used as the raw material.

The difference from the use of the apparatus shown in FIG. 4 lies in the destination of transfer of the molten salt separated from Ti particles in the separation step and the treatment at that destination. Thus, the molten salt decreased in Ca concentration as being separated in the high-temperature decanter 19 is sent to the Ca-removing region 30 in the Ca removing/concentrating vessel 28 a provided in the Ca removing/concentrating unit 28 via Route La, as shown in FIG. 6, while the adherent molten salt separated from Ti particles in the separation vessel 22 is sent to the Ca-concentrating region 29 in the Ca removing/concentrating vessel 28 a via Route Lb.

Here, a voltage below the decomposition voltage for CaCl₂ is applied via the electrode plate 33 and electrode plate 34 so that the electrode plate disposed on the molten salt side in the Ca-removing region 30 may serve as a positive (+) electrode against the electrode plate 34 disposed on the molten salt side in the Ca-concentrating region 29.

This voltage application causes a molten Mg—Ca alloy 32 existing in the vicinity of the contact surface with the molten salt in the Ca-removing region 30 to function as a negative (−) electrode relative to the molten salt side (+electrode side) in the Ca-removing region 30, so that the dissolved Ca transfers to the molten Mg—Ca alloy 32 side, as indicated by the arrows given in the Ca removing/concentrating vessel 28 a in FIG. 6, and absorbed into the alloy. As a result, the dissolved Ca in the Ca-removing region 30 is removed and the Ca concentration in the Mg—Ca alloy 32 increases.

On the other hand, the molten Mg—Ca alloy 32 in the vicinity of the contact surface with the molten salt in the Ca-concentrating region 29 functions as a positive (+) electrode relative to the molten salt side (−electrode side) in the Ca-concentrating region 29. Therefore, the Ca in the molten Mg—Ca alloy 32 transfers to the molten salt side in the Ca-concentrating region 29, so that the Ca concentration in the Ca-concentrating region 29 increases.

In this way, by applying a voltage below the decomposition voltage for CaCl₂ to the electrode plate 33 in the Ca removing/concentrating vessel 28 a, it becomes possible to remove the dissolved Ca in the Ca-removing region 30 and, at the same time, increase the dissolved Ca concentration in the Ca-concentrating region 29. In addition, by using the Ca removing/concentrating unit 28 whose principal constitution/feature is shown in FIG. 6, it becomes possible to carry out these simultaneous treatments with ease using such an apparatus that is very facile in configuration and constitution/feature.

That the voltage to be applied is selected at a level lower than the decomposition voltage for CaCl₂ is to avoid the formation of Ca in case of decomposition of CaCl₂.

As for the electrodes for the voltage application mentioned above, it is recommended that iron or a like metal be used as the negative (−) electrode and a graphite electrode or a like insoluble electrode as the positive (+) electrode, like in the case of the electrode bars to be mounted on the above-mentioned Ca recovery vessel 6 (shown in FIG. 4 and FIG. 5).

In the Ca removing/concentrating unit 28, the Ca removal and concentration treatments are carried out simultaneously in the manner mentioned above, and the Ca dissolved in the molten salt in the Ca-removing region 30 is removed and the Ca concentration in the molten salt in the Ca-concentrating region 29 increases.

Route Lc disposed between Route La and Route Lb is the route for taking a balance of the amounts of the portion of the molten salt in the Ca-removing region 30 and those in the Ca-concentrating region 29. Thus, since the amounts of the molten salt separated in the high-temperature decanter 19 is overwhelmingly larger than those of the adherent molten salt separated in the separation vessel 22, Route La and Route Lb themselves are not able to take a balance of the amounts of the molten salt in the Ca-removing region 30 and those in the Ca-concentrating region 29, with a result that it is not possible anymore to continuously carry out the Ca removal and concentration treatments in the Ca removing/concentrating unit 28. Therefore, part of the molten salt separated in the high-temperature decanter 19 is sent, via Route Lc, to the Ca-concentrating region 29 so that the treatments mentioned above may be carried out continuously.

The molten salt deprived of Ca in the Ca removing/concentrating unit 28 is sent to the “electrolysis step”. Since that molten salt has been deprived of Ca, a so-called back reaction, namely the reaction of Ca in the molten salt with chlorine formed by electrolysis, is suppressed and the Ca formation by electrolysis can be executed efficiently.

The molten salt in the Ca-concentrating region 29 is returned to the reduction step. Since the remaining Ca in the adherent molten salt has been concentrated and has an increased Ca concentration, it is effective in enhancing the efficiency of the TiCl₄ reduction reaction.

The Ca formed by electrolysis in the electrolysis step is introduced, either alone or together with the molten salt, into the reaction vessel via the “return step”.

In the Ti or Ti alloy production process according to the present invention, it is desirable that, in the Ca removing/concentrating unit 28, the dissolved Ca in the Ca-removing region 30 be removed and, at the same time, the concentration of the dissolved Ca in the Ca-concentrating region 29 be increased at an “applied voltage lower than 3.2 V (namely, voltage below the decomposition voltage for CaCl₂)”. The lower limit to the applied voltage is not particularly specified. For effective Ca removal, however, it is preferable that the applied voltage be not lower than 0.01 V.

FIG. 7 is a schematic representation of another example of the constitution/feature of an apparatus to be used in carrying out the Ti or Ti alloy production process including a Ca removing/concentrating step according to the present invention.

This apparatus is a modification to the above-mentioned apparatus shown in FIG. 6 that is made by further providing a regulating vessel 25 for introducing thereinto the molten salt in the electrolytic cell 3 and bringing the same into contact with a Ca supply source to thereby render the Ca concentration in that molten salt constant, and for thereafter feeding the resulting molten salt into the reaction vessel 1.

The Ti or Ti alloy production process according to the present invention is desirably one in which “the molten salt increased in Ca concentration in the electrolysis step is introduced into a regulating vessel provided with a Ca supply source and the molten salt is brought into contact with the Ca supply source to render the Ca concentration in the molten salt constant, and thereafter, the resulting molten salt is sent to the reduction step”.

The Ti or Ti alloy production apparatus according to the present invention is an apparatus to be used in carrying out the above-mentioned Ti or Ti alloy production process including such a Ca removing/concentrating step, and the constitution/feature thereof and the functions of the respective structural elements are as schematically shown in FIG. 6. When this apparatus is used, the Ti or Ti alloy production process including such a Ca removing/concentrating step according to the present invention can be properly carried out.

In carrying out the above-mentioned “2: Ti or Ti alloy production process including a Ca recovery step” and “3: Ti or Ti alloy production process including a Ca removing/concentrating step”, it is possible to employ a mode of embodiment such that a chlorination step is added and the formed TiCl₄ is used as a raw material for the Ti formation reaction within the reaction vessel.

More specifically, chlorine (Cl₂) is generated as byproduct on the anode side upon electrolysis of the molten salt in the above-mentioned electrolysis step, and this Cl₂, when allowed to react with titanium oxide (TiO₂), gives TiCl₄. Therefore, the Cl₂ formed on the anode side with the progress of electrolysis of the molten salt is caused to react with a titanium ore to form TiCl₄ and this TiCl₄, after purification by distillation, is used as the raw material in the production of Ti or a Ti alloy.

In the case of Ti alloy production, the Cl₂ formed on the anode side is caused to react with a mixture of TiO₂ and metal oxide(s) because at least one metal is added as an alloy element, to give a metal chloride mixture including TiCl₄, which can be used as the raw material.

By employing such a mode of embodiment, it becomes possible to effectively utilize the by-product Cl₂ obtained in the electrolysis of the molten salt and recycle the Cl₂ in the production process.

INDUSTRIAL APPLICABILITY

The method of removing/concentrating a metal-fog-forming metal in a molten salt according to the present invention can remove the metal-fog-forming metal dissolved in a molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts from one portion of the molten salt mixture and transfer the same to the other portion of the molten salt mixture for increasing the concentration thereof in that mixture. This method can be carried out easily and properly using the apparatus according to the present invention.

Therefore, the method of removing/concentrating a metal-fog-forming metal in a molten salt and the apparatus therefor can be expected to be utilized as one of means for treating a molten salt in various industrial fields in which metal-fog-forming metal containing molten salts, the metal-fog-forming metal being such as Ca or Na, are handled. In particular, they can be effectively utilized in the production of Ti by Ca reduction.

The Ti or Ti alloy production process according to the present invention makes it possible to remove (recovery) the Ca dissolved in the molten salt to be fed to the electrolytic cell and thereby aim at enhancing the Ca formation efficiency in electrolyzing the molten salt. Further, the process can contribute not only to increase the Ca concentration in the molten salt to be fed to the reaction vessel, simultaneously with the Ca removal (recovery), while allowing to enhance the efficiency of the Ca formation, but also to enhance the efficiency of the TiCl₄ reduction reaction and, when a regulating vessel is used, the process can make it possible to inhibit the fluctuation of and maintain at a high level the Ca concentration in the molten salt to be fed into the reaction vessel. Furthermore, the process makes it possible to continuously treat a large quantity of a CaCl₂-containing molten salt and increase the feeding rate of Ca to the reaction vessel and thereby it becomes possible to efficiently carry out the Ca formation in electrolysis of the molten salt and the reduction of TiCl₄ and carry out the operation stably on a commercial scale.

Therefore, the Ti or Ti alloy production process according to the present invention and the production apparatus according to the present invention which makes it possible to carry out that process easily and properly can be effectively utilized in the production of Ti or a Ti alloy by reduction with Ca. 

1. A method of removing/concentrating a metal-fog-forming metal in a molten salt, comprising: holding “a molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts” (hereinafter, generally referred to as a “molten salt”) with the metal-fog-forming metal being dissolved therein both in a metal-fog-forming metal concentrating and removing regions (hereinafter, referred to as a “concentrating region” and a “removing region”, respectively) in a metal-fog-forming metal removing/concentrating vessel, the concentrating and removing regions being apart from each other; holding, in contact with the molten salt held in each of the concentrating and removing regions, a metal-fog-forming metal containing molten alloy; and applying a voltage below a decomposition voltage for the metal-fog-forming metal containing molten salt so that an electrode plate on the molten salt side within the removing region may serve as a positive (+) electrode against the molten salt side within the concentrating region, thereby causing the metal-fog-forming metal dissolved in the molten salt within the removing region to be absorbed into the molten alloy to result in the concentration thereof being decreased and, at the same time, increasing the concentration of the metal-fog-forming metal dissolved in the molten salt within the concentrating region.
 2. The method of removing/concentrating a metal-fog-forming metal in a molten salt according to claim 1, wherein the metal-fog-forming metal is Ca and the metal-fog-forming metal-containing molten salt is a Ca-containing molten salt.
 3. The method of removing/concentrating a metal-fog-forming metal in a molten salt according to claim 1, wherein the metal-fog-forming metal is Ca and the metal-fog-forming metal-containing molten salt is CaCl₂.
 4. The method of removing/concentrating a metal-fog-forming metal in a molten salt according to claim 3, wherein the voltage to be applied is lower than 3.2 V.
 5. A process for producing Ti or a Ti alloy which comprises: a reduction step in which a CaCl₂-containing molten salt with Ca being dissolved therein is held in a reaction vessel and said Ca in the molten salt is allowed to react with a TiCl₄-based metal chloride to form Ti particles or Ti alloy particles in that molten salt; a separation step in which said Ti particles or Ti alloy particles are separated from the molten salt inside or outside the reaction vessel; an electrolysis step in which the molten salt taken out of the reaction vessel is electrolyzed to form Ca for increasing the Ca concentration in the molten salt; a return step in which said Ca formed by the electrolysis is introduced, either alone or together with the molten salt, into the reaction vessel; and a Ca recovery step in which while the molten salt separated in the separation step and to be sent to the electrolysis step is kept in contact with a molten alloy containing Ca and Mg, a voltage below a decomposition voltage for CaCl₂ is applied so that an electrode bar on the molten alloy side may serve as a negative (−) electrode and an electrode bar on the molten salt side as a positive (+) electrode to thereby cause the Ca dissolved in the molten salt to be absorbed into the molten alloy, and the molten salt reduced in Ca concentration is sent to the electrolysis step.
 6. A process for producing Ti or a Ti alloy which comprises: a reduction step in which a CaCl₂-containing molten salt with Ca being dissolved therein is held in a reaction vessel and said Ca in the molten salt is allowed to react with a TiCl₄-based metal chloride to form Ti particles or Ti alloy particles in the molten salt; a separation step in which said Ti particles or Ti alloy particles are separated from the molten salt inside or outside the reaction vessel; an electrolysis step in which the molten salt taken out of the reaction vessel is electrolyzed to form Ca to thereby increase the Ca concentration in the molten salt+a return step in which said Ca formed by electrolysis is introduced, either alone or together with the molten salt, into the reaction vessel; and a Ca removing/concentrating step in which a voltage below a decomposition voltage for CaCl₂ is applied so that an electrode plate on the molten salt side within a Ca-removing region serving to hold the molten salt separated in the separation step and to be sent to the electrolysis step may serve as a positive (+) electrode against an electrode plate on the molten salt side within a Ca-concentrating region separated from the Ca-removing region and serving to hold the molten salt to be sent to the reduction step, and the molten salt reduced in Ca concentration in the Ca-removing region is sent to the electrolysis step and the molten salt increased in Ca concentration in the Ca-concentrating region is sent to the reduction step.
 7. The process for producing Ti or a Ti alloy according to claim 5, wherein the voltage to be applied is lower than 3.2 V.
 8. The process for producing Ti or a Ti alloy according to claim 5, wherein the molten salt increased in Ca concentration in the electrolysis step is introduced into a regulating vessel provided with a Ca supply source, brought into contact with the Ca supply source to thereby render the Ca concentration therein constant and thereafter sent to the reduction step.
 9. The process for producing Ti or a Ti alloy according to claim 5, wherein the molten alloy increased in Ca concentration as a result of absorption of Ca in the Ca recovery step is used as the whole or part of the Ca supply source for a regulating vessel provided with a Ca supply source and intended for introducing the molten salt in the electrolytic cell thereinto and bringing the same into contact with the Ca supply source to thereby render the Ca concentration in the molten salt constant and, thereafter, feeding the molten salt into the reaction vessel.
 10. An apparatus for removing/concentrating a metal-fog-forming metal in a molten salt, comprising a metal-fog-forming metal removing/concentrating vessel including: a metal-fog-forming metal concentrating region (hereinafter, referred to as a “concentrating region”) that holds “a molten salt mixture consisted of one or more of metal-fog-forming metal containing molten salts” (hereinafter, generally referred to as a “molten salt) increased in the concentration of the metal-fog-forming metal dissolved therein; a metal-fog-forming metal removing region (hereinafter, referred to as a “removing region) that is separated from said concentrating region and intended for holding the metal-fog-forming metal containing molten salt decreased in the concentration of the metal-fog-forming metal dissolved therein as a result of application of a voltage below a decomposition voltage for the metal-fog-forming metal-containing molten salt via an electrode plate so that it may serve as a positive (+) electrode against the molten salt side within the concentrating region; and a molten alloy holding region for holding a metal-fog-forming metal containing molten alloy in contact with the molten salt both in the concentrating and removing regions.
 11. The apparatus for removing/concentrating a metal-fog-forming metal in a molten salt according to claim 10, wherein the metal-fog-forming metal is Ca and the metal-fog-forming metal containing molten salt is a Ca-containing molten salt.
 12. The apparatus for removing/concentrating a metal-fog-forming metal in a molten salt according to claim 10, wherein the metal-fog-forming metal is Ca and the metal-fog-forming metal containing molten salt is CaCl₂.
 13. An apparatus for producing Ti or a Ti alloy, comprising: a reaction vessel for holding a CaCl₂-containing molten salt with Ca being dissolved therein and reacting a TiCl₄-based metal chloride that is fed into the molten salt with said Ca to cause the formation of Ti particles or Ti alloy particles therein; a separation means for separating said Ti particles or Ti alloy particles formed in the molten salt from the molten salt; an electrolytic cell which holds the molten salt after separation of said Ti particles or Ti alloy particles therefrom and is equipped with an anode and a cathode for carrying out electrolysis in the molten salt to form Ca on the cathode side; a return means for introducing the Ca formed by electrolysis, either alone or together with the molten salt, into the reaction vessel; and a Ca recovery means (a) for applying, while keeping the molten salt separated by the separation means and to be fed to the electrolytic cell in contact with a molten alloy containing Ca and Mg a voltage below a decomposition voltage for CaCl₂ so that an electrode bar on the molten alloy side may serve as a negative (−) electrode and an electrode bar on the molten salt side as a positive (+) electrode to thereby cause the Ca dissolved in the molten salt to be absorbed into the molten alloy and (b) for sending the molten salt reduced in Ca concentration to the electrolytic cell.
 14. An apparatus for producing Ti or a Ti alloy, comprising: a reaction vessel for (a) holding a CaCl₂-containing molten salt with Ca being dissolved therein and (b) reacting a TiCl₄-based metal chloride, which is fed into the molten salt, with the Ca to cause the formation of Ti particles or Ti alloy particles therein; a separation means for separating said Ti particles or Ti alloy particles formed in the molten salt from the molten salt; an electrolytic cell which holds the molten salt after separation of said Ti particles or Ti alloy particles therefrom and is equipped with an anode and a cathode for carrying out electrolysis in the molten salt to form Ca on the cathode side; a return means for introducing the Ca formed by electrolysis, either alone or together with the molten salt, into the reaction vessel; and a Ca removing/concentrating unit which comprises (a) a Ca-removing region for holding the molten salt separated in the separation step and to be fed to the electrolysis step and (b) a Ca-concentrating region being separated from the Ca-removing region and serving to hold the molten salt to be sent to the reduction step, the unit serving to apply a voltage below a decomposition voltage for CaCl₂ so that an electrode plate on the molten salt side in the Ca-removing region may serve as a positive (+) electrode against an electrode plate on the molten salt side in the Ca-concentrating region and, further, serving to send the molten salt thus reduced in Ca concentration in the Ca-removing region to the electrolysis step and the molten salt thus increased in Ca concentration in the Ca-concentrating region to the reduction step.
 15. The apparatus for producing Ti or a Ti alloy according to claim 13, further comprising a regulating vessel provided with a Ca supply source and intended for introducing the molten salt in the electrolytic cell thereinto, bringing the same into contact with the Ca supply source to thereby render the Ca concentration in the molten salt constant and, thereafter, feeding the molten salt into the reaction vessel.
 16. The apparatus for producing Ti or a Ti alloy according to claim 13, comprising a regulating vessel provided with a Ca supply source and intended for introducing the molten salt in the electrolytic cell thereinto, bringing the same into contact with the Ca supply source to thereby render the Ca concentration in the molten salt constant and, thereafter, feeding the molten salt into the reaction vessel, wherein the molten alloy increased in Ca concentration in the Ca recovery means is used as the whole or part of the Ca supply source in the regulating vessel.
 17. The process for producing Ti or a Ti alloy according to claim 6, wherein the voltage to be applied is lower than 3.2 V.
 18. The process for producing Ti or a Ti alloy according to claim 6, wherein the molten salt increased in Ca concentration in the electrolysis step is introduced into a regulating vessel provided with a Ca supply source, brought into contact with the Ca supply source to thereby render the Ca concentration therein constant and thereafter sent to the reduction step.
 19. The apparatus for producing Ti or a Ti alloy according to claim 14, further comprising a regulating vessel provided with a Ca supply source and intended for introducing the molten salt in the electrolytic cell thereinto, bringing the same into contact with the Ca supply source to thereby render the Ca concentration in the molten salt constant and, thereafter, feeding the molten salt into the reaction vessel. 